Depends entirely on the aircraft. For Boeing 767, the RAT is hydraulic but is used to power an auxiliary generator at least on ETOPS aircraft. On all Bombardier aircraft (CL65, CRJ & GX) the RAT is an Air Driven Generator (ADG) that's prime use is to power the main pump for Hydraulics 3 and also provides power to all Essential & Battery Bus units up to approximately 15 kVA. In most cases the prime purpose is to provide hydraulics to power the flight controls power drive units as there is no direct manual link to the surfaces (think fly by wire with one channel prestressed cable).
Don't forget that on most aircraft the primary backup is not the RAT but, in practice, the APU which can provide at least 25 kVA. The reason it's not relied on is that it is nigh impossible to certify an APU as "Essential" such that it can be guaranteed to start & run over the entire flight envelope.